Abstract. During the field campaign ZEPTER-2 in autumn 2008 whole air samples were
collected on board a Zeppelin NT airship in the planetary boundary layer
(PBL) and the lower free troposphere (LFT) over south-west Germany using the
ZEppelin Based Isotope Sampler (ZEBIS). These samples were analysed with
respect to volatile organic compound (VOC) mixing ratios and stable carbon
isotope ratios using a gas chromatograph combustion isotope ratio mass
spectrometer (GC-C-IRMS). In this study we present results for toluene, one
of the major anthropogenic pollutants, which emphasise the viability of
isotope ratio measurements in VOC for atmospheric research, especially to
study VOC sources or to track both dynamical and chemical processes. In situ
measurements of CO mixing ratios on board the Zeppelin NT were used to
allocate the air samples either to the PBL or the LFT.

In the PBL we observed rather fresh emissions mixing into the background air.
We estimated a toluene source isotope ratio of
δ13C = −28.2 ± 0.5‰. Samples from the PBL and
the LFT were clearly distinguishable by means of their mixing ratio and
isotope ratio signatures. Using the concept of the effective kinetic isotope
effect, we were able to separate the effects of dilution processes and
photochemical degradation in the free troposphere. We calculated the
photochemical age of toluene in the atmosphere in two different ways using
isotope ratios and mixing ratios. The results differ strongly
in the PBL, probably due to mixing processes, but are compatible with each
other in the LFT. Here, they correlate with a slope of 0.90±0.31.